It is desirable to have an improved system and method for reducing resistance to flow in liquid reservoir extraction. An example of liquid reservoir extraction is depicted in FIG. 1, where an oil pumping apparatus extracts oil from an underground oil bearing formation via an oil well. The basics of an oil well are well described by Wikipedia, the internet free encyclopedia, from which the description below was derived with some minor changes.
A typical oil well, depicted in FIG. 2, is created by drilling a hole 5 to 30 inches (13-76 cm) wide into the earth with an oil rig turning a drill bit. After the hole, which may also be referred to as borehole or wellbore, is drilled, a metal pipe slightly smaller than the hole size (called a ‘casing’) is placed into the hole. The outside of the casing is then bonded and secured to the hole with cement. The casing provides structural integrity to the newly drilled wellbore in addition to isolating potentially dangerous high pressure zones from each other and from the surface.
With these zones safely isolated and the formation protected by the casing, the well can be drilled deeper (into potentially more-unstable and violent formations) with a smaller bit, and also cased with a smaller size casing. Modern wells often have 2-5 sets of subsequently smaller hole sizes drilled inside one another, each cemented with casing.
Tubing is then inserted inside the casing and used attached to a pumping apparatus in order to extract the oil from the ground. This tubing is often referred to as the production conduit.
The space between the casing and the tubing is called the annulus, as shown in FIGS. 3a and 3b. The oil well may include an assembly called a packer that is placed in the wellbore and activated so that it forms a seal between the tubing and the casing. The packer also serves to provide stability to the tubing. Although there are many different types of packers, a packer can be generally described as a downhole device used to isolate the annulus from the production conduit, enabling controlled production, injection or treatment. A typical packer assembly incorporates a means of securing the packer against the casing or liner wall, such as a slip arrangement, and a means of creating a reliable hydraulic seal to isolate the annulus, typically by means of an expandable elastomeric element. Packers are classified by application, setting method and possible retrievability.
When a new oil field first begins producing oil, Nature does most of the work. The natural pressures in the reservoir force the oil through the rock pores, into fractures, and up production wells. This natural flow of oil is called “primary production.” It can go on for days or years. But after a while, an oil reservoir begins to lose pressure, like the air leaving a balloon. The natural oil flow begins dropped off, and oil companies use pumps (like shown in FIG. 1) to bring the oil to the surface.
In some fields, natural gas is produced along with the oil. In some cases, oil companies separate the gas from the oil and inject it back into the reservoir. Like putting air back into a balloon, injecting natural gas into the underground reservoir keeps enough pressure in the reservoir to keep oil flowing.
Eventually, however, the pressure drops to a point where the oil flow, even with pumps and gas injection, drops off to a trickle. Yet, there is actually a lot of oil left in the reservoir. In many reservoirs, as many as 3 barrels can be left in the ground for every 1 barrel that is produced. In other words, if oil production stopped after “primary production,” almost ¾ths of the oil would be left behind. That's why oil producers often turn to “secondary recovery” processes to extract some of this remaining oil out of the ground.
A lot of oil can be left behind after “primary production.” Often, it is clinging tightly to the underground rocks, and the natural reservoir pressure has dwindled to the point where it can't force the oil to the surface.
One secondary recovery approach used by oil producers is to drill wells called “injection wells” and use them like gigantic hoses to pump water into an oil reservoir. The water washes some of the remaining oil out of the rock pores and pushes it through the reservoir to production wells. The process is called “waterflooding” and typically results in the recovery of an additional five to fifteen percent of the oil from a reservoir. Similar oil recovery techniques include steam flooding and CO2 flooding. Waterflooding and steam flooding are depicted in FIGS. 4a and 4b, respectively.
As such, 65% to 70% of the oil in a reservoir is left behind after primary production and secondary recovery are finished. That is the situation faced by today's oil companies. In the history of the United States oil industry, more than 160 billion barrels of oil have been produced. But more than 330 billion barrels have been left in the ground. Unfortunately, present methods are unable to extract most of this oil from the ground.
A key problem related to the limitations of primary production and secondary recovery methods is the flow resistance directly attributable to the damage that occurs to the near well formation (or near-wellbore area) as a result of the fractures in the formation being compressed due to the pressure of the ground pushing down on the formation while the pressure in the near well formation is lowered by the oil extraction process. Additional damage occurs from natural phenomena such as fines migration, clay swelling, scale formation, organic deposition, including paraffins or asphaltenes, and mixed organic and inorganic deposition. Induced damage includes plugging caused by foreign particles in the injected liquid, wettability changes, emulsions, precipitates or sludges caused by acid reactions, bacterial activity and water blocks. Generally, resistance to flow increases as damage to the near well formation occurs over time.
One stimulation treatment routinely performed on oil and gas wells in low-permeability reservoirs to counter the effects of damage is called hydraulic fracturing. Specially engineered liquids and significant amounts of water are pumped at high pressure and rate into the reservoir interval to be treated, causing a vertical fracture to open. The wings of the fracture extend away from the wellbore in opposing directions according to the natural stresses within the formation. Proppant, such as grains of sand of a particular size, is mixed with the treatment liquid keep the fracture open when the treatment is complete. Hydraulic fracturing creates high-conductivity communication with a large area of formation and is intended to bypass any damage that may exist in the near-wellbore area. Hydraulic fracturing, or fracking, is very costly and can be hazardous to groundwater as described in an Apr. 14, 2005 Telluride Daily Planet article by D. Dion entitled “Fracturing regs reach breaking point” from which key excerpts are provided below.
The process of hydraulic fracturing is used in almost all oil and gas drilling to stimulate production; liquids are injected underground at high pressure, and the geological formations fracture, allowing the oil or gas to be released. Some of the liquids remain trapped underground and are toxic enough to contaminate groundwater, according to the Oil and Gas Accountability Project (OGAP) report. “The EPA admits,” said the report's author Lisa Sumi, “that chemicals used in fracking can enter drinking water . . . at concentrations that pose a threat to human health.”
The draft EPA study, according to OGAP, showed that at least nine fracking chemicals, even when diluted with water, are still concentrated enough to pose a threat to human health: benzene, phenanthrenes, naphthalene, 1-methylnapthalene, fluorenes, aromatics, ethylene glycol and methanol. These chemicals have been linked to such health problems as cancer; liver, kidney, brain, respiratory and skin disorders; and birth defects. OGAP found that citizens from Colorado, New Mexico, Virginia, West Virginia, Alabama and Wyoming reported changes in water quality and pressure from hydraulic fracturing operations. The complaints were similar: murky or cloudy water, black or gray sediments, iron precipitates, soaps, black jelly-like grease, floating particles, diesel fuel or petroleum odors, increased methane in water, rashes from showering, gas taste and loss of water pressure.
Because existing primary production and secondary recovery methods leave behind the majority of the oil in a given reservoir and because current stimulation methods intended to bypass damage to near well formations are costly and potentially hazardous to groundwater, there is a need for an improved system and method for reducing resistance to flow in liquid reservoir extraction.